The present invention generally relates to distribution of gas and sensors for measurement of mass of gas flow in a duct. More particularly, the invention relates to achieving and maintaining accuracy of flow measurement as the gas flow undergoes varying configurations of flow patterns.
In some distribution systems for a gas such as air, a flow control valve may be positioned within a duct through which the air is distributed. For example, the control valve may be a butterfly valve. A flow sensor may be positioned downstream from the valve and signals from the sensor may be employed to vary an angle of opening of the butterfly valve. By varying the angle of opening of the valve responsively to the flow sensor, a desired mass flow of air may be maintained irrespective of air pressure that may develop upstream of the valve.
A flow control valve may introduce variations of flow configuration as air passes through it. In a butterfly valve, for example, a clockwise swirl pattern may be introduced at a large valve angle opening, while at a smaller valve angle opening, a counterclockwise swirl pattern may be introduced. Additionally, a degree of turbulence in the flow downstream of the valve may vary as a function of valve angle opening.
A typical mass gas flow sensor may employ a heated sensing device which may transfer heat into gas passing the sensing device. The heated sensing device may be maintained at a desired temperature with an adjacent electrical heater. Determination of magnitude of mass flow passing the sensing device may be achieved by measuring an amount of current needed to maintain the heated sensing device at the desired temperature.
Typically the heated sensing device is housed within a thermally conductive shell. As flow configuration varies, more or less gas may impinge on various locations on the shell, thus producing variations in an amount of heat transferred from the shell into the passing gas flow. This may lead to reductions in accuracy of the mass gas flow sensor.
As can be seen, there may be a need for a mass gas flow sensor that can produce accurate results irrespective of a presence of variations in gas flow configuration.